MUSEUM OF COMPARATIVE ZOOLOGY. 217 



No motion in the spermatozoa having been seen, their direct penetra- 

 tion into the vitelliis is necessarily beyond the observer's experience; 

 nor has anything been observed in the living egg at this early stage to 

 indicate that penetration had taken place. Nevertheless it is highly 

 probable, judging from the observations that have recently been made 

 in cases more favorable for the study of this phase of development, that 

 the substance of at least one of these thread-like spermatozoa is already 

 embraced within the vitellus at the time the egg is laid. 



The earliest indication observable in the living egg which can be 

 referred even indirectly to the presence of a spermatozoon occurs some 

 time after the formation of the second polar globule. 



Allusion has previously been made to the fact, that after the forma- 

 tion of what has been called the female pronucleus there appears a sec- 

 ond structure of similar aspect. The latter is situated at quite a distance 

 from the former, and is often more deeply imbedded in the vitellus, on 

 account of which it is usually less distinct (m. pn.. Figs. 65, 30). After 

 a time it is found nearer the female pronucleus, and in proportion as it 

 nears the latter it becomes larger and more easily seen. At length 

 the two lie side by side, but still continue to increase in size. I have 

 not been able to make out any constant difference in dimensions between 

 them ; it is therefore often quite impossible to say which is the female 

 pronucleus, and which the other body, unless they have been under ob- 

 servation for some time previous to their contact. The latter has 

 quite the same appearance as the female pronucleus. Its study in the 

 living egg is permitted only occasionally by an exceptional transparency 

 of the yolk. Under favorable circumstances its outline is seen to ex- 

 hibit slow changes of form such as have already been described for the 

 female pronucleus. In its growth it keeps pace with the latter. In 

 some cases considerable portions of their surfaces appear in contact. In 

 others, however, even when of great size, they are only very close to 

 each other, and do not touch. 



The meagre results to be obtained from the study of the living egg 

 are very well supplemented, in some points at least, by that which may 

 be learned by other means of investigation. The use of osmic acid is 

 more satisfactory than any other single method. I have not had the 

 same success in the use of acetic acid, since it does not cause the vitel- 

 lus to become as transparent, nor are the nuclear structures made so 

 conspicuous by it. To the fact that osmic acid was used much less 

 frequently than acetic acid is probably due my want of success in de- 

 tecting this nuclear body at an earlier stage in its formation. 



